The present invention relates to the chemical vapor codeposition of multiple elements and particularly of aluminum and one or more active (gettering) elements, such as Hf, Zr, Y, to form a protective aluminide coating bearing the active (gettering) element on a substrate in effective amount to improve coating oxidation/corrosion resistance.
It is well known to improve the oxidation and/or corrosion resistance of various nickel, cobalt and/or iron base superalloy gas turbine engine components by forming a protective aluminide diffusion coating thereon. One coating process used to form aluminide coatings on superalloy substrates involves a pack process (pack cementation) wherein the substrate to be coated is placed in a particulate bed of coating material (e.g. aluminum particulates, halide activator, and inert diluent) and heated in a retort to an elevated coating temperature to generate an aluminum halide coating gas that reacts with one or more substrate elements (e.g. Ni for a nickel base superalloy substrate) to form the aluminide coating (e.g. nickel aluminide) on the substrate. U.S. Pat. Nos.3,544,348, 3,961,098, 4,070,507, and 4,132,816 disclose pack coating processes for forming aluminide coatings.
Another coating process used to form aluminide coatings on superalloy substrates involves a chemical vapor deposition process (CVD process) wherein the substrate to be coated is placed in a reactor, heated to elevated coating temperature, and a coating gas comprising aluminum halide (e.g. Al trichloride or subchlorides) and a carrier gas, such as hydrogen, is introduced to the reactor to deposit aluminum on the substrate for reaction therewith to form the aluminide coating thereon. The Gauje U.S. Pat. No. 3,486,927 represents an early effort directed to a CVD coating process.
The CVD process can be used to form aluminide coatings including additional alloying elements in the coating. For example, U.S. Pat. No. 2,772,985 discloses formation of binary Alxe2x80x94B, Alxe2x80x94Si, Alxe2x80x94Ti, and Alxe2x80x94Zr coatings on Mo substrates. The patented process first vapor phase deposits Al on the Mo substrate and then reacts the deposited Al with the alloying element (B, Si, Ti or Zr) deposited from a second coating gas. The aluminum halide coating gas and alloying element halide coating gas are generated in separate parallel generators to provide separate coating gas streams. The process does not involve codeposition of the coating elemental constituents on the substrate.
U.S. Pat. No. 4,687,684 relates to CVD formation of oxidation resistant Si and Cr-enriched aluminide diffusion coatings on a superalloy substrate. The patent provides for sequential introduction of the aluminum halide coating gas and the silicon or chromium halide coating gas to the coating reactor. The patent does not involve codeposition of the coating elemental constituents on the substrate.
U.S. Pat. No. 5,015,502 discloses-a CVD coating process for forming aluminide coatings containing a gettering element (e.g. Y) by chlorination of a source of aluminum containing Y and one or more of Si, Cr, Co, etc. A preferred source is described as an Alxe2x80x94Yxe2x80x94Si alxe2x80x94loy.
Aluminum and silicon have been intermittently codeposited on a heated superalloy substrate by CVD such that a silicon-modified aluminide coating is formed. In particular, low silicon contents of trace to 8 weight % are desirable to improve the oxidation/corrosion resistance of the aluminide coating. Since typical silicon precursors, such as silicon tetrachloride and dichlorosilane, are much less stable than aluminum trichloride, a short pulse (e.g. 7.5 minutes) of the silicon halide or silane coating gas is provided intermittently (e.g. once every two hours) during the continuous CVD aluminum deposition to obtain the desired silicon concentration in the coating. Since no intermetallic compounds form between aluminum and silicon and the coating is applied at high temperature, silicon can be deposited on the substrate from pulses of silicon tetrachloride or dichlorosilane and then it diffuses during the aluminizing cycle to give a homogenous silicon modified aluminide coating.
If Al and a reactive element (such as Hf, Zr and Y) which form stable intermetallic compounds with Si are continuously codeposited while Si is intermittently codeposited by the aforementioned CVD pulsing technique, then the resulting modified aluminide coating will be heterogenous in that the pulses of silicon coating gas produce layers in the coating that contain a large volume fraction of silicon rich intermetallic compounds. Such a coating structure wherein the majority of the reactive element (gettering/active element) exist as intermetallic compound layers does not exhibit the superior oxidation resistance of a modified aluminide coating wherein the reactive element is uniformly dispersed throughout the aluminide coating.
Thus, there is a need for the continuous deposition of aluminum and a reactive element, such as Hf, Zr, and/or Y, with or without Si to obtain a more homogenous coating for oxidation resistance.
It is an object of the invention to provide a CVD method and apparatus for codepositing multiple elements on a substrate in a manner to overcome disadvantages discussed above.
It is another object of the invention to provide a CVD coating method and apparatus for forming aluminide coatings wherein aluminum and one or more reactive elements for surface active superalloy substrate impurities (e.g., S, P, etc.) are codeposited on a substrate.
It is still another object of the invention to provide a CVD coating method and apparatus for forming an aluminide coating including silicon and one or more reactive elements for surface active superalloy substrate impurities wherein aluminum, silicon and the reactive element(s) (e.g. Hf, Zr, Y) are codeposited on the substrate.
It is still a further object of the invention to provide a coated superalloy article having an aluminide coating thereon including one or more reactive elements for surface active superalloy substrate impurities uniformly distributed in a region of the coating or throughout the coating for improved oxidation resistance.
The present invention provides in one embodiment method/apparatus for forming on a substrate a chemical vapor deposited (CVD""ed) aluminide coating including one or more reactive elements, such as Hf, Zr, and Y, dispersed therein by virtue of codeposition of Al and the reactive element(s) during CVD coating. In a particular embodiment of the invention wherein the reactive element, such as Hf and Zr, can be halogenated (e.g. chlorinated) at relatively low temperatures (e.g. less than 600xc2x0 C.), the invention comprises flowing a first halide precursor gas in a carrier gas in contact with a first source comprising aluminum disposed outside a coating retort to generate an aluminum halide first coating gas, flowing a second halide precursor gas in an inert carrier gas in contact with a second source comprising a reactive element (e.g. Hf or Zr) disposed outside the coating retort to generate a second halide coating gas of the reactive element, and introducing the first and second coating gases concurrently into a coating retort in which the substrate at elevated temperature is disposed to codeposit Al and the reactive element on the substrate for coating formation.
The first coating gas preferably is formed by flowing hydrogen chloride in a hydrogen carrier gas in contact with Al particulates heated to a reaction temperature to form aluminum trichloride. The second coating gas preferably is formed by flowing hydrogen chloride in an inert carrier gas in contact with particulates comprising an a reactive element selected from the group consisting essentially of Hf and Zr heated to a reaction temperature to form the tetrachloride thereof.
In another particular embodiment of the invention wherein the reactive element, such as Y, can be halogenated (e.g. chlorinated) only at relatively high temperatures (e.g. greater than 600xc2x0 C.), the invention comprises flowing a halide precursor gas in a carrier gas in contact with a first source comprising aluminum disposed outside a coating retort to generate an aluminum halide coating gas, flowing the first coating gas into the coating retort in contact with a second source comprising a reactive element (e.g.Y) disposed inside the retort and heated to the necessary reaction temperature to convert a portion of the first coating gas to a halide coating gas of the reactive element, and contacting the coating gases concurrently with the substrate at elevated temperature in the retort to codeposit Al and the reactive element on the substrate for coating formation.
In this embodiment of the invention, prior to contacting the substrate, the unconverted portion of the first coating gas is flowed in contact with a secondary source comprising an aluminum alloy in the coating retort to increase the activity of aluminum therein. For example, the first coating gas can comprise aluminum trichloride in a hydrogen/inert carrier gas. The activity of aluminum in the unconverted portion of the first coating gas is increased by forming aluminum subchlorides by contact with a secondary source comprising an aluminum alloy (e.g. Alxe2x80x94Co or Alxe2x80x94Cr) in the retort downstream of the gettering element source. This provides the desired amount of Al deposition on the substrate.
In this embodiment of the invention, the first coating gas can include a tetrachloride of the reactive element (such as hafnium or zirconium tetrachloride) generated outside the coating retort so as to codeposit two reactive elements on the substrate.
Also in this embodiment of the invention, a silicon halide coating gas can be introduced into the coating retort in a manner to by-pass the reactive element source so as to concurrently contact the substrate with the other coating gases present to codeposit Al, Si, and the reactive element(s) on the substrate for coating formation.
The present invention provides in a further embodiment method/apparatus for codepositing first and second elements on a substrate. This embodiment involves flowing a halide precursor gas in a carrier gas in contact with a first source comprising a first element disposed outside a coating retort to generate a first halide coating gas, flowing the first coating gas into the coating retort in contact with a second source comprising a second element disposed inside the coating retort to convert a portion of the first coating gas to a halide coating gas of the second element, and contacting the first and second coating gases concurrently with the substrate at elevated temperature in the coating retort to codeposit the first and second elements on the substrate.
The present invention provides a coated substrate comprising a substrate and a CVD""ed aluminide diffusion coating thereon having a dispersion of one or more reactive elements (e.g. Hf, Zr, Y) in a region (e.g. inner coating region or outer coating region) or throughout the entire coating by virtue of the reactive element(s) being codeposited with aluminum on the substrate. The aluminide coating also may include Si uniformly distributed therein by codepositon with Al and the reactive element(s).